Diagnostic X-ray equipment is well known for so called non-invasive examination. Equipment is available for industrial as well as medical applications. A most important element of such equipment is the generator of X-rays which is most typically a high vacuum tube with the capability of generating an electron beam and accelerating the beam toward a high speed rotating target where the impact produces X-rays which pass out of the vacuum envelope and are collimated and directed toward the patent or sample being studied. For standard X-ray diagnostic tubes, electric fields of 150 KV/inch to 300 KV/inch are employed which are produced in conjunction with DC voltages of 75 to 150 KV. Typically the distance between the cathode and the rotating target is on the order of 0.5 to 1 inch. It is known in such standard purpose X-ray tubes to superimpose electron beams produced from more than one filament onto the same focal spot on the anode target. In such standard purpose X-ray tubes this focussing is accomplished using a pair of cathode cups employing two and three slot designs. Typically, the slots have been machined grooves which form two cups which are symmetrically displaced about an axis. The cathode filaments are normally mounted adjacent the intersection of the smallest and next smallest slot. When the filament is mounted inside of the smallest slot, its emission is reduced because of space charge effects. The dimensions of the slots and the distance between the center of the slots to enable focusing of the beams from adjacent cups to a single spot has heretofore required at least 0.5 inch of anode to cathode spacing.
Mammography X-ray diagnostics is a special application for which a specific mammography X-ray tube has become standard. Specifically, the mammography tube is very much shorter in overall length than the standard X-ray tubes. The mammography tube is particularly designed to be able to have its X-ray exit port very close to the patient's breast to obtain the highest resolution and contrast picture possible.
Superimposition of electron beams from adjoining cathode cups has not been heretofore achieved in mammography X-ray tubes because the slot dimensions necessary in standard two slot cathode cup configurations required the center of the slots to be too far apart to allow the electron beams to become superimposed over the shorter anode to cathode distance employed in mammography tubes. For mammography tubes, the DC voltage employed is only 25 to 30 thousand volts. Because the shorter anode to cathode distances employed in these tubes, i.e. less than 0.3 inches, the fields are 110 KV/inch to 130 KV/inch.
In view of the above problems, currently designed mammography X-ray tubes are not capable of providing high intensity electron beams and are generally considered cathode emission limited. This requires the typical mammograph examination for large spot applications to take 1-2 seconds and for small spot, high resolution examinations to take approximately 5 seconds. The high resolution, 5 second examination time, introduces significant opportunity for picture blurring due to patient movement or other mechanical and environmental vibrations. Specifically, cathode filaments in mammography tubes with 0.1 mm focii typically could deliver only 25-30 ma and for a typical 0.3 mm focii could deliver only approximately 100 ma. Since the high voltage employed is 25 KV, the target anodes are not fully loaded. A three to four inch rotating anode can handle these power levels at 3000 RPM. Since the mammography X-ray tubes are capable of rotating their target anodes at speeds up to 9000 RPM, and the power handling capacity at this higher speed is 70% greater than at 3000 RPM, a technique to provide greater electron beam intensity can be accommodated by the existing mammography X-ray tube design by increasing the anode speed.